In the past, the p-type electrodes in flip-chip nitride semiconductor light emitting elements have been used a structure comprised of silver or a silver alloy. Because silver very efficiently reflects the light produced by the light emitting layer of a light emitting element, such an element is able to emit light of high brightness.
When silver is used for the electrode material on the p side, however, part of the silver electrode surface has to be exposed for the purpose of connection with external components and so forth, and this causes and promotes the migration of silver ions, which changes the silver at the interface with the nitride semiconductor layer, and can lead to problems such as decreased light emission intensity due to the light produced by the light emitting layer not being reflected as efficiently by the electrode, and decreased service life due to the movement of silver to the other electrode, which causes short-circuiting.
One measure that has been taken to deal with this problem is to completely cover the silver electrode with an-electrode material that does not contain silver, and form a protective film over this, in order to prevent the silver electrode surface from being exposed. However, if a heat treatment is performed after electrode formation, or depending on the conditions of this heat treatment and so forth, it is sometimes impossible to adequately suppress the diffusion of the silver into the electrode material that contains no silver, which means that the migration of silver cannot be prevented.
In response to this problem, a method has been proposed for preventing the migration of silver by disposing an SiO2 film having a plurality of through holes between the silver electrode and the electrode material containing no silver, and electrically connecting the silver electrode and the electrode material containing no silver through this holes (see Patent Document 1, for example).
Other proposals include an LED in which a metal layer is formed over a silver layer on a p-type nitride semiconductor layer, and a dielectric layer is formed on part of the surface of the silver layer (see Patent Document 2, for example), a semiconductor light emitting element in which a p-type electrode is formed from a laminate of a contact layer and a reflecting layer made of a silvery white metal disposed over this contact layer (see Patent Document 3, for example), a semiconductor element having an-electrode formed from a first metal layer connected to a contact layer on the p-type semiconductor side, and a second metal layer that covers at least the side face of the first metal layer and the surface of the contact layer not covered by the first metal layer, in which the first metal layer is formed from silver (Ag), and the second metal layer from vanadium (V) and aluminum (Al) or titanium (Ti) and gold (Au) (see Patent Document 4, for example), a light emitting element equipped with a first thin-film metal layer comprised of cobalt (Co), nickel (Ni), or an alloy of these between a p-type nitride semiconductor layer and a positive electrode (see Patent Document 5, for example), and so forth.
Patent Document 1: JP-2003-168823-A
Patent Document 2: JP-H11-186599-A
Patent Document 3: JP-H11-191641-A
Patent Document 4: JP-H11-220171-A
Patent Document 5: JP-2000-36619-A
With Patent Document 1, however, because an SiO2 film is disposed between a silver electrode and an electrode material containing no silver, even though electrical connection is ensured by the through-holes, a problem is increased contact resistance between the two electrodes.
Also, although the migration of silver to the electrode material containing no silver is suppressed by physically blocking off the electrode with the SiO2 film, this does not effectively prevent the migration of silver to the nitride semiconductor layer, so this still does not mitigate the decrease in the service life of the light emitting element or the decrease in light emission intensity attributable to the migration of silver.
Furthermore, the suppression of silver migration cannot be considered satisfactory in any of these publications, and there is a need for a light emitting element that better prevents this migration, which in turn affords better semiconductor element reliability and higher yield, as well as a semiconductor element with more efficient light take-off.